NEW YORK — New York Mayor Michael Bloomberg was present, in the rain on Grand Army Plaza, for the opening of the sculpture installation “Circle of Animals/Zodiac Heads,” by Chinese artist Ai Weiwei. Bill T. Jones, the choreographer, was there, as well as artist Julian Schnabel and Agnes Gund, philanthropist and former president of the Museum of Modern Art.

Ai Weiwei was not there. Ai, 53, has been missing since early last month, when he was taken away by Chinese authorities.

“The very fact that we do not know where he is or when he will be released is very disturbing,” the mayor said.

The artwork, a dozen monumental bronze heads depicting the animals of the Chinese zodiac, stood in the Pulitzer Fountain behind the podium, in a row facing Central Park. The Sheep’s mouth had poked through its bubble wrap in transit, and now there were bluish flecks of oxidation there.

A matching set is supposed to go on display in London on May 12, and Berlin is hosting a show of Ai’s work involving dead trees; in all, this month Ai’s art should be visible in about a half-dozen cities around the globe. This week marked the end of his installation at the Tate Modern in London, a sea of 100 million handmade porcelain replicas of sunflower seeds.

The unimaginable scale of the labor that went into the seed project told one story about modern China. The fact that the Tate decided to keep the public from trampling through the seeds, as planned, for fear of raising harmful dust clouds, ended up telling another story about modern China.

The bronze heads, likewise, have picked up layers of significance. They are oversize versions — 10 feet tall on their pedestals and 800 pounds — of the collection of bronze zodiac-animal heads looted by French and British troops from Beijing’s Old Summer Palace in 1860, during the Second Opium War. Those originals, scattered around the globe, have become a symbol of China’s humiliation and powerlessness before the West. Reclaiming them, whenever one comes up for auction, is a Chinese nationalist project.

And now they are standing, a full set, just outside the Plaza Hotel. Ai has filled in the gaps created by the five missing originals with more expressive and elaborate treatments: an exuberantly spiny Dragon, a Rooster with a gnarled and angry-looking comb. All that’s missing this time around is the artist — a humiliation inflicted by China on itself.

Ai’s detention has been a shock in a way that the disappearances of Chinese rights activists and lawyers — an ever-growing list — has not. Jailing a reformer is one thing; jailing a famous artist is another.

So, in mid-April, in a semi-flash protest pulled together on Facebook, artists turned up and sat on chairs (a tribute to an Ai installation of 1,001 antique Chinese chairs in Germany) across from the Chinese consulate in New York and at China’s embassies and consulates around the world. By the West Side Highway, there were wicker chairs and folding camp chairs and a milk crate labeled “chair.”

“One person disappeared could be all of us disappeared,” one protester said. It sounded aspirational, in a country where artists can provoke as they please, but among the chairs was an empty one, an antique wooden Chinese seat, with the characters “yan nian” — “prolong life”— carved in the back.

It was Ai’s own chair, brought by his friend Alanna Heiss, who had known him in the decade he spent in New York, from 1983 to 1993.

Ai “considers himself both a citizen of the world and a New Yorker,” Bloomberg said in his speech at the unveiling of the bronzes.

But Ai is also a citizen of China. As a child, he followed his father, the Communist revolutionary poet Ai Qing, into exile and forced labor in the far reaches of the country after the party turned on him. “He helped his father clean latrines,” Heiss said.

Now the son has been caught in another reversal. He was taken into custody April 3 at Terminal 3 of Beijing’s airport — an immense, coppery-roofed feat of triumphal architecture designed by Britain’s Norman Foster. The terminal opened in 2008, in time for the Beijing Olympics, a symbolic gateway to an open, prosperous, cosmopolitan People’s Republic.

The international spirit has been in retreat lately. Ai was a designer of another monument of 2008 — Beijing’s Olympic stadium, the “Bird’s Nest.” By the time the Games arrived, however, he had denounced the project as a “fake smile” covering the country’s failures and repression. He turned his attention to investigating and commemorating the deaths of thousands of schoolchildren in that year’s Sichuan earthquake — victims of collapsed buildings traceable to shoddy construction.

He provoked the system, in a seemingly open-ended piece of performance art, by pretending it was reasonable and accountable that an ordinary citizen had the right to dissent. He filmed and recorded and tweeted the results — the paperwork, the demolition of his Shanghai studio, his beating by Chengdu police and subsequent emergency brain surgery — up through April 3.

The silence since has become its own statement. There was a report in state media that Ai was being investigated for “economic crimes,” but no specific accusations have been made. More telling was an editorial in the state-run English-language Global Times denouncing Ai — in the stilted, atavistic language of the old, closed Chinese state — as “a maverick of Chinese society” who would someday “inevitably touch the red line” of the law.

Under Chinese law, the police must state the charges against a detained person within seven days but may wait as long as 30 days if there is an ongoing investigation. The opening of the zodiac-heads exhibit — postponed two days because of the death of Osama bin Laden — ended up coinciding with Ai’s 31st day in captivity. The red line was moving.

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